I think think genetics sets up a kind of "base" for behaviour and for behavioural response to the environment. Even your vision changes how you think. But personality is a widely social phenomenon, even though brain chemistry and genetics are both involved. There's a very complicated interplay that goes on between genetics and the other ways we learn, adapt and add to the world. We're more or less stuck with our genetics, but we can even change some genetic factors that go into how we interact with the world. A specific way at looking at personality is to see how it changes with sleep, diet (including drugs and sunlight, which are part of the body's diet in a broader sense) and learning (especially engaged, challenging learning that promotes plasticity of neurons and changes in neuronal activity). We can alos look at how our habits change. Addiction and obsession-compulsion (the latter can potentially be cured, explained at the * below) are special cases, where genetics play a larger role, but if you have a habit and you want to lose it, try to replace it with another habit for about 2-3 weeks and see if the old habit comes back. If it does, you may have an addiction and not just a bad habit. But replacing old habits with new ones generally works, so long as it occupies the same perceived need.
Your personality is influenced by both genetics and the environment, but your brain and body are original to you, so you can actually change your genetics to some extent (not all of your genetics, but certain things related to your personality). It's very important to pay attention to the fact that you can think new thoughts and change the world. Yes, you rely on your genetics and experience to do this, but you have choice and ability, and this means your personality isn't wholly determined. In fact, people who tend to think in novel ways tend to have more dynamic personalities, and they tend to be more intelligent, than average. And what's really cool is that even if someone doesn't tend to think in new ways (especially imaginatively), by
trying to think in new ways after conscously and programmatically deciding to do so, a person can not only alter his or her personality, but he or she can widen its range and bcome more intelligent (physically, this can mean changes in the brain like increased neural connections, chemical changes affecting mood, and even changes of activity and size in some areas of the brain). However, the opposite can happen if someone smokes, drinks heavily, takes (even some prescription) drugs, doesn't exercise, or doesn't challenge his or her mind. More reason to eat well, rest, exercise, read, learn new things, think postively, etc

These things also change your brain lifespan and your overall longevity.
*Fixing obsession-compulsion: ins ome disorders, there's no fix, but for minor disorders, if you simply change how you think about something, it will actually be manifested in the plasticity of your neurons. So say that you wake up at night and feel the need to make sure you turned off the oven. Every time you try to do it, tell yourself, especially aloud, "I don't need to do this , because my fear isn't real. I won't set the house on fire and I know better". In most cases, after a few days or weeks, you won't do it anymore.